1. Field of the Invention
This invention relates to phase-shift keying (PSK) and differential phase-shift keying (DPSK) in telecommunication. More specifically, it relates to the use of a rhomb beam splitter for converting a phase-keyed signal to an intensity-keyed signal in PSK and DPSK.
2. Description of the Prior Art
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases is used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal (such a system is termed coherent).
U.S. Ser. No. 11/729,004 and Ser. No. 12/174,628, hereby incorporated by reference, describe an implementation of PSK in an optical hybrid that includes a 50/50 un-polarized beam splitter, a folding prism, a beam shifter, a spacer and a phase shifter. From a signal beam S and a reference beam L directed into the optical hybrid, four outputs, S+L, S−L, S+jL and S−jL, are produced. The phase difference between the two interfering components of each output beam produced by the S and L beams in the optical hybrid is θ+0, θ+90, θ+180, or θ+270 degrees, where θ is the phase difference of the signal beam with respect to the reference beam.
Instead of using bit patterns to set the phase of the signal wave, in differential phase-shift keying (DPSK) modulation the patterns are used to set changes in the phase of the signal. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying. DPSK can be significantly simpler to implement than ordinary PSK because there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (i.e., it is a non-coherent scheme).
In telecommunication technology, DPSK has been implemented with various interferometric devices. In particular, the Mach-Zehnder interferometer and various embodiments of the Michelson interferometer have been used, such as described in co-owned U.S. application Ser. No. 11/360,959, Ser. No. 11/485,653, and Ser. No. 12/505,483, in a continuing effort to improve signal quality and manufacturing costs. The present invention teaches the advantageous use of the so-called rhomb beam splitter in various interferometer configurations for a variety of optical communication applications.